Method of improving the operational characteristics of cold cathode devices having crossed electric and magnetic fields



3,383,149 METHOD OF IMPROVING THE OPERATIONAL CHARACTERISTICS May 14, 1968 P. J. BRYANT ETAL OF GOLD CATHODE DEVICES HAVING CROSSED ELECTRIC AND MAGNETIC FIELDS i" i 2 QBK EL l l I l l l l 358 .Emizu 2 Pressure (Torr) INVENTORS. Paul J. Bryan? Charles M. Gosse/in United States Patent 3,383,149 METHOD OF IMPROVING THE OPERATIONAL CHARACTERISTICS OF COLD CATHODE DE- VICES HAVING CROSSED ELECTRIC AND MAGNETIC FIELDS Paul J. Bryant, Prairie Village, Kans., and Charles M.

Gosselin, Kansas City, Mo., assignors to Midwest Research Institute, Kansas City, M0,, a corporation of Missouri Filed .Fune 2?, 1965, Ser. No. 467,947 6 Claims. (Cl. 316-3) ABSTRACT OF THE DISCLOSURE The work function of the cathode of a cold cathode device employing crossed electric and magnetic fields is lowered by subjecting the cathode to cesium while the device is in an evacuated environment. This is effected by decomposing a cesium compound in a vacuum systern communicating with the device, the free cesium released upon decomposition being highly mobile due to the partial vacuum condition and, therefore, contacting the cathode as the cesium travels throughout the system. An increase in the sensitvity of thedevice is produced by the treatment which extends the low pressure limit of operation of magnetron vacuum gauges and renders getter ion pumps readily restartable after a current interruption.

The invention described herein was made in the performance of work under a NASA contact and is subject to the provisions of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 426; 42 U.S.C. 2451), as amended, and relates to a method of improving the operational characteristics of cold cathode devices having crossed electric and magnetic fields by cathode cesiation.

With rapid developments in space technology in recent years, an increasing need for ultrahigh vacuum systems capable of simulating the conditions of outer space in the laboratory has arisen. Accordingly, it has become necessary to provide a suitable pressure-responsive gauge capable of indicating the very low pressures encountered in ultrahigh vacuum systems.

Cold cathode magnetron vacuum gauges arein widespread use for this purpose; however, such gauges have inherent limitations and disadvantages. First, the gauge has a cutoff point at pressures between 1O and 10* torr, thereby rendering the gauge unsatisfactory for lower pressure readings. Secondly, the response of the gauge in the unsaturated, non-linear response range thereof varies from gauge to gauge and thus is not uniform; manifestly, this causes wide deviations in the low pressure limit of operation. Thirdly, striking characteristics, i.e., initiation of pressure readings when the magnetron tube is excited at low pressures, are poor in the nonlinear response range of the gauge as several minutes are often required to obtain a reading.

In addition to the need for suitable gauges, it is also necessary to provide pumping techniques capable of evacuating a chamber to very low pressures. Getter ion pumps are available as a relatively simple pumping means suitable for applications where a lower pressure limit of approximately 10* torr is suificient, but restriking or restarting of these pumps in the lower pressure range operational mode is difficult.

It is, therefore, the broad objective of this invention to provide a method of improving the operational characteristics of cold cathode devices having crossed electric and magnetic fields to obviate the disadvantages discussed above.

Another important object of the invention is to provide an improved magnetron vacuum gauge having uniform response characteristics and an extended low pres sure reading capability.

As a corollary to the last-mentioned object, it is a further aim of this invention to provide a method of treating the cathode of the magnetron tube of a cold cathode magnetron gauge which will effect the aforesaid uniformity of response and increased pressure range and, additionally, substantially improve the striking characteristics of the gauge and decrease the pressure at which nonlinear response commences.

It is another important object of the instant invention to provide a method of treating getter ion pumps to render the same capable of nearly instantaneous restarting in the low pressure operational mode thereof.

In the drawing:

FIGURE 1 is a simplified, diagrammatic perspective view of a magnetron vacuum gauge in an environmental chamber; and

FIG. 2 is graph showing the response of a typical magnetron gauge as compared with a gauge treated in accordance with the instant invention.

FIGURE 1 shows the basic components of a cold cathode, magnetron tube, such components comprising a cylindrical anode It) and a metallic cathode electrode 12 extending along the axis of the cylinder defined by anode 10. Opposed magnetic poles 14 and 16 direct a constant magnetic field axially of anode 10, the lines of flux (not shown) being directed from pole 14 to pole 16. The anode is perforated as indicated at 18 so that gas molecules may readily pass between the cathode and the anode. A DC source 20 and a micromicroammeter 22 complete the gauge structure, source 20 being connected between anode 1t and ground, while meter 22 is connected between the cathode 12 and ground.

The cathode end structures, supplemental electrodes, and the envelope of the tube are not shown in FIG. 1 in order that the operation of the tube might be clearly illustrated. The tube is diagrammatically portrayed within an environmental chamber 23 of a vacuum system. Those skilled in the art will appreciate that couplings are provided on the envelope structure to communicate the magnetron tube with the vacuum system so that the tube will be subjected to the same pressure as the system.

Operation of the gauge is illustrated by three arrows 24 emanating from cathode 12 which represent field emission electrons from the cathode. The field emitted electrons travel toward anode 10 by virtue of the electric field established between the cathode and the anode by source 2%. However, the mean free paths of the elec trons are considerably lengthend due to the presence of the fixed magnetic field which causes the electrons to traverse hypercycloidal paths within the anode cage.

The arrows 26 adjacent the inner surface of anode 10 represent the electrons as they traverse their orbits. One such electron is represented at 28 and shown as it collides with a gas molecule 30. The collision may cause ionization of the molecule to form a positive gas ion which, in turn, would be attracted by the negative cathode 12 along a path 32. An ionizing collision also causes the release of an additional electron which is free to travel to anode 10, thereby creating an avalanche effect which, above a certain pressure, causes saturation of the space charge between the cathode and the anode. Besides the field emission current, the positive ions bombarding cathode 12 cause secondary emission from the cathode which aids in the establishment of the saturated space charge condition.

Referring to FIG. 2, the response curve 34 illustrates the typical response characteristics of a cold cathode mag- '3 netron gauge. Response curve 34 has a linear segment 36 and a nonlinear segment 38. The break point between the two segments is illustrated by the horizontal line 40 which intersects the ordinate axis at approximately 3X10 amperes. This represents the ion current in the magnetron at the break point between linear and nonlinear response.

Another current level of interest is the cutoff current of the gauge, which is represented by the horizontal line 42 intersecting the ordinate axis at approximately 9 10 ampercs. Both the abscissa and ordinate axes are plotted logarithmically, the abscissa representing the pressure within the magnetron envelope.

It will be appreciated from viewing FIG. 2 that the cutoff point or threshold of gauge response occurs at approximately X10 torr and thus represents the lower limit of gauge usefulness. It should be understood that the characteristic curve 34 is representative of a typical magnetron gauge and, more particularly, is the average response determined experimentally of a Redhead gauge Model 552 manufactured by NRC Equipment Corporation, a subsidiary of National Research Corporation of Newton, Mass. In actual practice, the extrapolated logarithmic slope of the nonlinear segment 38 varies considerably from gauge to gauge and may, for example, be quite steep and shift the cutoff point of the gauge to pressures as high as torr.

The response curve 44 illustrates the response characteristics of a cold cathode magnetron gauge after treat ment of the same by the process of the instant invention, which will now be described in detail. Note that the linear segment 46 of the response characteristic has shifted leftwardly or toward the ordinate axis, the break point between the linear segment 46 and the nonlinear, curved segment 48 being maintained at a value of approximately 3X1O amperes. However, this break current value now corresponds to a pressure of approximately 3.5 1O- torr, in contrast to a pressure of approximately 7 10 torr for an untreated gauge. Thus, the sensitivity of the gauge has been increased by approximately a factor of two through the use of the treatment of the instant invention.

The treatment comprises introducing a decomposable cesium compound near the envelope of the gauge and then selectively heating that region of the vacuum system to the decomposition temperature of the compound. It is believed that the results achieved by the instant invention are caused by free cesium atoms reacting with the magnetron cathode. A high partial vacuum is maintained within the gauge during heating of the compound. Thus, the cesium atoms are quite mobile and readily travel throughout the vacuum system thereby rendering the exact location of the compound in the system uncritical. For example, a glass vessel 50 containing the compound may be communicated with chamber 23, a mercury diffusion pump 52 being commonly employed to create the partial vacuum condition. Heating of the vessel 50 then causes the emission of cesium which travels throughout the system by molecular flow or surface flow over the system conduits.

Any decomposable cesium compound capable of yielding cesium upon decomposition thereof is suitable for use as a cesium source. Examples include cesium nitrate (CsNO cesium carbonate (Cs CO and various cornpcunds of cesium and antimony, including CS Sb. Cesium nitrate is particularly convenient for use as the source since it decomposes at temperatures above 414 C., and

However, a sensitivity increase of this magnitude causes an undesirable current background equivalent to approximately a 2 10 torr reading. Manifestly, background noise of this magnitude would render the gauge inoperable at lower pressures.

If, however, the cathode is only subjected to a moderate cesium treatment, it is found that the characteristic curve shown at 44 is obtained without the production of undesirable background current. As set forth above, the sensitivity of the gauge is increased approximately by a factor of two rather than by the maximum factor obtainable. It may be noted that the nonlinear segment 48 of the improved response characteristic reaches the cutoff current of 9X1O amperes at a pressure of approximately 8 1O- torr, resulting in a substantial extension of the low pressure limit of gauge operation.

It is also found that the above treatment renders the response of the gauge uniform, i.e., regardless of the response characteristic prior to the treatment, the characteristic after treatment is as shown by response curve 44. The extrapolated, midrange logarithmic slope of the nonlinear segment 48 is between approximately 1.3 and 1.4, while the extrapolated, inidrange logarithmic slope prior to the treatment averaged approximately 1.6 to 1.7 but was variable over a considerable range and unpredictable until a particular gauge was tested and calibrated.

Additionally, the striking time of the treated gauge upon initiation of gauge operation in the nonlinear response range thereof is substantially increased. At pressures of approximately 10* torr, untreated gauges may require from approximately one minute to twenty minutes before ion current flow is achieved and meter 22 produces an ion current indication. However, treated gauges respond nearly instantaneously at this pressure.

Cathode 12 may be conveniently subjected to the proper cesium treatment by introducing the selected decomposable compound into the system and effecting generation of cesium for approximately 10 to 15 minutes. Only a small amount of the compound is required; for example, if cesium nitrate is utilized, three to five granules thereof is adequate. This will cause the cathode to be heavily treated with cesium.

Reduction of the amount of cesium allowed to remain united with the cathode surface is then achieved by heating the magnetron to a temperature of 325 to 400 C. for a period of two to three hours. This drives off the excess cesium and leaves only a sutlicient amount of cesium united with the cathode surface to effect the desired sensitivity increase set forth above.

Besides utilizing the cesium treatment to improve the operational characteristics of magnetron vacuum gauges, the process also substantially improves the operation of other cold cathode devices having crossed electric and magnetic fields, such as getter ion pumps.

A getter ion pump is similar in structure to the cathode, anode, and magnetic pole arrangement shown in FIG. 1, except that the cathode of the pump is in the form of a pair of spaced plates disposed at respective ends of the cylindrical anode 10. The cathode is composed of titanium to increase the gettering action and hense the pumping rate. The operation of such pumps is based on phenomena similar to that as discussed hereinabove with referense to FIG. 1, the work of Dushman, S., and I. M. Lafr'erty, Scientific Foundations 0 Vacuum Technique, second edition, pp. 175475, incorporated herein by reference as may be required for a full and complete description of the structure and operation of getter ion pumps.

The pump treatment is identical to that as described above, the cesium compound being introduced into the vacuum system of the pump and decomposition effected after the pump is operated for a sufficient period of time to achieve high vacuum conditions. However, unlike the treatment of the magnetron cathode, the pump cathode may be subjected to as heavy a cesium treatment as desired, since the presence of background current is not 53 detrimental to the utilization of the phenomena to execute a pumping function.

The pump, after the cesium treatment of the instant invention, may be readily restarted should a current interruption occur after the pump has reduced system pressure to a level within the lower pressure range operational mode thereof. Conventional untreated pumps often may not restrike until system pressure rises above the low pressure operational mode of the pump which, of course, is time consuming and necessitates repumping of the system to the desired pressure level.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. A method of improving the operational characteristics of a cold cathode device having crossed electric and magnetic fields, said method comprising the step of subjecting the cathode of the device to cesium.

2. A method of improving the operational characteristics of a magnetron vacuum gauge, said method comprising the step of subjecting the cathode of the magnetron to cesium of .a sufiicient quantity to increase the sensitivity of the magnetron by approximately a factor of two in the linear response range thereof.

3. -A method of improving the operational characteristics of a magnetron vacuum gauge, said method comprising the steps of:

subjecting the cathode to the magnetron to cesium; and

limiting the final quantity of said cesium permitted to unite With the cathode to an amount capable of increasing the sensitivity of the magnetron in the linear response range thereof by only approximately a factor of two.

4. A method of improving the operational characteristics of a cold cathode device having crossed electric and magnetic fields and disposed in an environmental chamher, said method comprising the steps of:

introducing a decomposable cesium compound into said chamber capable of yielding cesium upon decomposition thereof;

evacuating said chamber;

decomposing said compound in said chamber after evacuation thereof to thereby produce said cesium; and thereafter subjecting the cathode of the device to the cesium by permitting the latter to disperse in said chamber.

5. A method of improving the operational characteristics of a cold cathode device having crossed electric and magnetic fields and disposed in an environmental chamber, said method comprising the steps of:

introducing a decomposable cesium compound into said chamber capable of decomposing at an elevated temperature and yielding cesium;

evacuating said chamber;

heating said compound to at least said temperature after evacuation of said chamber to thereby produce said cesium; and thereafter subjecting the cathode of the device to the cesium by permitting the latter to disperse in said chamber.

6. A method of improving the operational characteristics of a magnetron vacuum gauge disposed in an environmental chamber, said method comprising the steps of:

introducing a decomposable cesium compound into said chamber capable of yielding cesium upon decomposition thereof; evacuating said chamber; decomposing said compound in .said chamber after evacuation thereof to thereby produce said cesium;

thereafter subjecting the cathode of the magnetron to the cesium by permitting the latter to disperse in said chamber; and

limiting the final quantity of said cesium permitted to unite with said cathode to an amount capable of increasing the sensitivity of the magnetron in the linear response range thereof by only approximately a factor of two.

References Cited UNITED STATES PATENTS 2,754,442 7/1956 Boutry et a1 313230 X 2,835,835 5/1958 Boutry et al. 313230 X 2,884,550 4/1959 Lalferty 313-7 2,941,099 6 /1960 Picard et al. 3137 3,051,868 8/1962 Redhead 315-408 3,157,819 11/1964 Krohn 315-111 RICHARD H. EAN-ES, 111., Primary Examiner.

JAMES W. LAWRENCE, Examiner.

C. R. CAMPBELL, Assistant Examiner. 

